>When I click on ellipsis of the anchor property of a container, I get message
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>The Anchor property editor has been registered
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>On another container I don't get this message. What does it mean?

Editors are supposed to be registered in the builders.dbf somewhere in or below home() directory, but only once per class that they cover. I haven't really read the source of anchor builder (as I don't see why would I, it just works :), but I assume it's autoregistering. Look for builder.dbf, open it and look at the last records.